By Tony Chang SEOUL, Dec. 29 (Yonhap) -- North Korea said Tuesday that it was investigating an American who crossed into the country from China to draw attention to human rights abuses there, Pyongyang's first apparent response since the activist entered the country.

Robert Park, 28, recently entered North Korea to call international attention to the North's human rights record, according to fellow activists. The communist North is believed to maintain several concentration camps where tens of thousands of political prisoners are reportedly being held.

"A U.S. citizen has been detained by authorities after illegally entering the DPRK through the DPRK-China border on Dec. 24," a brief report from the North's Korea Central News Agency (KCNA), monitored in Seoul, said.

The DPRK is short for Democratic People's Republic of Korea, the North's official name.

The KCNA report, which did not disclose further details regarding the detainee, said a "relevant organ" was conducting the investigation.

According to the activists, who claim to be members of a coalition of more than 100 groups focused on assisting North Korean defectors and addressing the country's notorious human rights conditions, its leader Robert Park, a Korean-American, crossed the frozen Tumen River at around 5 p.m. last Friday.

The 28-year-old from Tuscon, Arizona carried a letter with him for the North's leader calling for the opening of the tightly-controlled border for the shipment of food and medical goods and the closure of all concentration camps housing political prisoners, they added.

The U.S. State Department on Monday expressed concern that the North may have detained the activist.

In March, two American journalists who crossed into North Korea from China while working on a story about human trafficking spent more than four months in jail for illegal entry.

The journalists, Laura Ling and Euna Lee, were sentenced to 12 years of hard labor but were freed as part of a diplomatic mission spearheaded by former U.S. President Bill Clinton in August.

No accurate data on the North's human rights situation is available as the communist nation strictly controls traffic across its border. But the U.N. and global human rights groups say that citizens there have no freedom of speech and dissidents suffer torture and even execution without trial.